Cycloidal mass spectrometer employing crossed uniform magnetic and electric fields



'Feb. M, 1969 D. ANDREW 3,427,449

CYCLOIDAL MASS SPECI'ROMETER EMPLQYING CROSSED UNIFORM MAGNETIC ANDELECTRIC FIELDS Sheet Filed June 29. 1966 INVENTOR. DEREK ANDREW AGE 7Feb. 11, 1969 5, ANDREW 3,427,449

CYCLOIDAL MASS SPECTROMETER EMPLOYING CROSSED UNIFORM MAGNETIC ANDELECTRIC FIELDS Filed June 29, 1966 Sheet 2 of5 j 16 TN 5 7 b/ TINVENTOR. DEREK ANDREW M fC-l AGE " Filed June 29, 1966 Feb. 11, 1969 0.ANDREW 3,427,449

CYCLQIDAL MASS SPECTROMETER EMPLOYING CROSSED UNIFORM 4 MAGNETIC ANDELECTRIC FIELDS Sheet 3 015 INVENTOR. DEREK ANDREW AGENY United StatesPatent Ofitice 3,427,449 Patented Feb. 11, 1969 27,736/ 65 U.S. Cl.250-413 Claims Int. Cl. H01j 39/34; 301d 59/44 ABSTRACT OF THEDISCLOSURE A cycloidal mass spectrometer employing crossed uniformmagnetic and electric fields in which ions cover cycloidal paths, theelectric field being produced by a high resistance wire wound around aregion in which ions cover their paths, a potential difference beingapplied to the ends of the wire.

The present invention relates to a mass spectrometer, more particularlyto a so-called cycloidal mass spectrometer, in which ions leave an ionsource through a gap and cover cycloidal paths in crossed uniformelectric and magnetic fields and finally reach a collecting electrodethrough a gap.

In the known cycloidal mass spectrometers, the uniform electric field isobtained by providing a plurality of parallel fiat plates each appliedto a fixed voltage and provided with apertures for passing the ions.

In these mass spectrometers, it is extremely difficult to attain andmaintain the high-vacuum required for a satisfactory operation. Due tothe flat metal plates constituting large trapped surfaces, it isdifficult to achieve a sufficient outgassing even if a prolongedbaking-out process is carried out at 450 C. Moreover, each platerequires a separate electrical connection through the vacuum envelope.

The object of the invention is to provide an improvement of cycloidalmass spectrometers obtained by a novel structure for producing theuniform electric field in a mass spectrometer which permits readilyoutgassing of the spectrometer.

In a cycloidal mass spectrometer, the uniform electric field is producedaccording to the invention by means of a wire which is helically woundaround a region in which the ions cover their paths, voltage differencebeing applied between the ends of this wire.

The overall electric field can be produced by means of more than onewire helix, for example, by means of two wire helices each located onone side of a central plate. In a favorable embodiment of the invention,the wire of each helix is wound around posts having a metal coresurrounded by a ceramic sleeve. Adjacent at least one of these posts, aleaf spring lying between the core and the sleeve pushes the sleevetowards the outer side of the wire helix. Thus, the Wire is stretchedand slackening of the wire due to heat developed by the resistance ofthe wire is neutralized.

Use may be made, for example, of four posts arranged as the ribs of arectangular parallelepipedon.

A cycloidal mass spectrometer having one or more wire helices by meansof which the electric field is produced may be readily outgassed andrequires a minimum number of electrical connections for producing theelectric field.

The invention will now be described more fully with reference to theaccompanying drawing, in which:

FIG. 1 is a perspective view of part of a mass spectrometer,

FIG. 2 is a sectional view of one of the posts around which the wirehelix is wound,

FIG. 3 is a schematic sectional view of FIG. 1 taken on the axis of thewire helix and at right angles to the magnetic field in which a few ionpaths are outlined.

The structure shown in FIG. 1 is mounted on a flange and is locatedwithin a vacuum envelope consisting of a tube of stainless steel. Theflange, the vacuum envelope and various electrical connections are notshown in the figure, since these form no part of the invention.

The structure includes a central plate 1, which supports the ion source2 having a gap (3, FIG. 3) in its side (not shown in FIG. 1) from whichthe ions emanate. The ion beam can reach through an aperture 4 in thecentral plate 1 and through a gap 5 the collecting electrode (6, FIG. 3)behind the gap 5. A plate 7 is secured by four posts 8, 9, 10 and 11 toa detachable frame 12 clamped to the plate 1. Since 12 is detachable,the ion source is readily accessible.

In FIG. 2, one of the posts is shown on an enlarged scale. This postconsists of a metal core 13 and a ceramic sleeve 14 provided withnotches 15. 16 serves for spacing purposes. The core 13 is bolted to theplate 7 and the frame 12 but the sleeve is capable of performing arotation and a limited translation. The sleeve 14 is spaced apart fromthe core 13 by a leaf spring 17. Two diagonally opposite posts areconstructed in this manner. The other pair of posts each consist of ametal core, a ceramic sleeve serving the purpose of 16 and the outersleeve with notches which is not capable of performing a translation. Awire having a diameter of 0.025 mm. and a resistance of 2200 Q/m. iswound around the four posts 8, 9, 10 and 11. In FIG. 1, this wire helix18 is partly broken away for the sake of clarity. The wire extendsthrough the notches spaced apart by 0.5 mm. The expansion coefiicient ofthe wire is equal to that of the plate 7 and of the frame 12 so that thewire is not displaced during outgassing. Slackening of the wire due tothe heat developed by its resistance is counteracted by the leafsprings. This structure is capable of withstanding electric fields of 10v./m. The ceramic sleeves are coated with a thin layer of tin oxidehaving a resistance of the order of 10 9/ per square, which value issufficiently high to prevent the potential distribution established bythe wire from being disturbed and sutficiently low to prevent surfacecharge from being concentrated on the insulators.

FIG. 1 shows a second wire helix 19. This helix is mounted between theplate 20 and the plate 1 like the first wire helix between the plate 5and the plate 1. The beginning of the first wire helix is connected tothe plate 5 having a negative potential. The end of the first helix andthe beginning of the second helix are connected to the earthed plate 1.The end of the second helix is connected to the plate 20 having apositive voltage. Thus, only three electrical connections through thevacuum envelope are required for producing a uniform electric fieldhaving a direction parallel to the axis of the helices. Moreover,uniformity deviations may be compensated for toa certain extent, sincethe plate 7 is adjustable independently of the earthed frame 12.

FIG. 3 is a schematic sectional view of FIG. 1 taken on the axis of theturns and at right angles to the magnetic field. The figure indicatesthe direction of the magnetic field H of the order of 2,000 gauss andthat of the electric field E and outlines a few paths of ions focussedin the gap 5' before the collecting electrode 6.

What is claimed is:

1. A cycloidal mass spectrometer employing substantially perpendicularmagnetic and electric fields comprising an ion source, a collectorelectrode for ions leaving said source, means for producingsubstantially perpendicular magnetic and electric fields in which ionsfrom said source cover cycloidal paths before reaching the collectorelectrode, said electric field means comprising a high resistance wirehelically wound around a region in which ions cover their paths, saidhelically wound wire producing a uniform electric field in a directionparallel to the axis of the helically wound wire when a potential isapplied between the ends of the wire, said helically wound wire beingwound about posts, having ceramic sleeves, at least one of which isspring loaded away from the center of the helix to take up wire slackcaused by ohmic heating during operation of the spectrometer, and meansto apply a potential difference between the ends of said wire to producean electric field.

2. A mass spectrometer as claimed in claim 1 in which the overallelectric field is produced by more than one Wire helix.

3. A mass spectrometer as claimed in claim 2, in which the overallelectric field is produced by two wire helices each located on oppositesides of a central plate having an aperture through which said ionstravel.

4. A mass spectrometer as claimed in claim 3, in which each wire helixis wound around four posts arranged like ribs of a rectangularparallelepipedon.

5. A mass spectrometer as claimed in claim 3, in which the ceramicsleeve of each post is provided with notches which hold the wire inplace and is coated with a thin layer of tin oxide.

References Cited UNITED STATES PATENTS 2,769,910 11/1956 Elines.

RALPH G. NILSON, Primary Examiner.

S. C. SHEAR, Assistant Examiner.

